In the prior art, a pearlescent pigment prepared by coating a flaky substrate (such as mica flakes) with a metal oxide having a high refractive index was known. As substrates desirable for use in this kind of pigment, synthetic materials having stable quality, such as flaky iron oxide, flaky titanium oxide, and aluminum-doped flaky iron oxide, have been prepared and marketed.
Furthermore, aluminum oxide having high hardness has been used as abrasion-resistant material, ceramic material, coating pigment, abrasive, or the like, and flaky aluminum oxide having improved properties has been prepared.
However, these kinds of aluminum oxide have a problem in that they are not suitable for use as a flaky substrate for pearlescent pigments because of their excessively small particle size small aspect ratio, strong tendency toward crystal twinning and aggregation, and poor dispersion in water. In addition, such aluminum oxide particles are difficult to coat with a metal oxide, because they are poorly dispersible in water and are composed of aluminum oxide alone. Even if coating is possible, the resulting coated particles are not suitable for use as a substrate for pearlescent pigments, because the metal oxide particles have a non-uniform thickness and size and are liable to aggregate. Moreover, the above-mentioned aluminum oxide particles have a disadvantage that they are usually produced by a hydrothermal process which needs an expensive high-pressure reactor.
In addition, studies have been conducted on a method of preparing flaky aluminum oxide using titanium oxide as an additive and a method of preparing flaky aluminum oxide using zinc oxide or tin oxide as an additive.
However, in the method that uses titanium oxide as an additive, the flaky aluminum oxide shows physical properties that are relatively suitable for use as a substrate for pearlescent pigments, but there is a problem in that it is difficult to obtain a flaky alumina crystal having a uniform particle size and excellent dispersibility, because an aging process is not performed after hydrolysis while a calcining process is not performed before crystallization. In addition, in the case in which zinc oxide or tin oxide is used as an additive, zinc oxide or tin oxide, which is present as an oxide, can be recognized as the cause of problems, several companies in highly developed countries restrict the contents of heavy metals in raw materials by the total dissolution method and regulate the use of some raw materials.